R. Baartman, R. Poirier, G. Dutto

Sept. 21, 2002

The cyclotron runs routinely at 200 µA at 500 MeV,
exceeding the design goal of 100 µA. We consider this to be the
activation limit. At this
level, it has reliability (delivered/scheduled beam time =)
~ 90%.

We have demonstrated the engineering limit
is at least
280 µA (couldn't go higher because nowhere to dump beam).

Electromagnetic stripping of H-->H0 ramps up
steeply from negligible at 450 MeV to 8% at 500 MeV. An Auxiliary
Accelerating Cavity (AAC)
was installed to get the beam through the
dangerous region more quickly. This is operating routinely since 2000 and
reduces the e-m stripping to 5%. If large amounts of current are to be
directed to radioactive beam production, this should be done at 450 Mev.

Between 30% and 50% of the injected beam is lost on the first few turns, of
which about half is lost on the centre post. This can be
as much as 100 Watts. Thermocouples showed high and strongly beam-dependent
readings. High temperatures -> more RF sparking. The solution,
implemented in the Jan. 2002 shutdown was to install a cooled beam
absorber against the centre post. (Before and after). We now feel 400 µA cw operation is
possible, but have insufficient beam-dump capacity to prove it.

Vertical focusing is caused by azimuthal modulation of the magnetic field (cut it in
sectors). This modulation cannot extend to small radius where "small"
means less than the magnet gap. TRIUMF has both a large magnet gap and a
small first turn. Therefore we must rely on electric focusing, which
only occurs if the beam is crossing the dee gaps while the electric field
is falling. The phase acceptance window is therefore from
0° to
40°. Moreover, phases where the slope is small (near 0°) are
most weakly focused. Space charge defocuses the beam and as a result, the
higher the local current, the more shifted to positive phases is the
acceptance window. See illustration. This ultimately results in a hard upper limit to the
beam current. We believe this limit to be around 500 µA
. (Reminder: 500 µA average in 36° bunches means the average
current in the bunch is 5 mA!. This is achieved by bunching.)

This requires some ISIS improvements as well as possibly additional cooled electrodes.

For setting up the cyclotron to high intensity, it is important to be able
to dump the beam somewhere. This will result in more reliable high
current ISAC operation. This will require an additional beam dump of
200 µA capability. If this is done at low energy (~100 MeV), it
only requires a 20 kW dump. Possible locations are on BL2C, and a new
BL5C.

To supply additional ISAC targets, as well as the current target stations,
as well as maintaining at least 150 µA
for muons and 50 µA for BL2C requires around
400 µA total beam.

This requires some ISIS improvements as well as possibly additional cooled electrodes.

For setting up the cyclotron to high intensity, it is important to be able
to dump the beam somewhere. This will result in more reliable high
current ISAC operation. This will require an additional beam dump of
200 µA capability. If this is done at low energy (~100 MeV), it
only requires a 20 kW dump. Possible locations are on BL2C, and a new
BL5C.